<!DOCTYPE HTML>
<html>
<body>
  <div id="elem" style="overflow-y:scroll;width:300px;height:200px;border:1px solid black">
    text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
    text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
    text text text text text text text text text text text text text text text text text text text text text text text text text text
  </div>

  요소에 <code>style="width:300px"</code>를 설정해 놓음
  <br>

  <button onclick="alert( getComputedStyle(elem).width )">alert( getComputedStyle(elem).width )</button>

</body>
</html>
